DESCRIPTION (Adapted from the Applicat's Abstract): Giardia is the major cause of infectious waterborne intestinal disease in humans. Nevertheless, little is known about the basic biology of Giardia, specifically, its mechanism of encystment, which is required for parasite transmission from one host to the next. Thus, the overall objective of this project is to provide detailed knowledge on pathway(s) of synthesis of the cyst wall-specific amino hexose, N-acetylgalactosamine (GalNAc), required for and specifically induced during Giardia encystment. This overall objective will be accomplished by completing the following specific aims: 1) Determination of the possible pathways of GalNAc synthesis using isotopic labeling of in vitro encysting Giardia trophozoites with known GalNAc precursors. 2) Characterization of key GalNAc synthetic enzymes with respect to their kinetics, substrate specificity, and sensitivity to known inhibitors. Completion of this project should give important information for pursuing the following long range goals: 1) to understand the mechanism of encystment and its regulation at the molecular level, 2) to develop new chemotherapy targeting GalNAc synthesis or other encystment pathways, and 3) to improve diagnostic methods for detecting Giardia in human and environmental samples.